Heat-shrinkable packaging films have been used for the packaging of a variety of products. Through the years, several efforts have been done in the technical field to improve the pack appearance by optimizing shrink and optical properties of the films, as well as to increase their stiffness in order to improve abuse resistance and machinability.
Concerning the shrinking properties, an ideal packaging film should have the correct balance of free shrink, maximum shrink tension and residual shrink tension in order to provide packages with an appealing appearance and a satisfactory functionality, which should be preserved under the most common packaging and storage conditions and over time for the entire package life.
Any deviation of the optimal values of said shrinking properties may be detrimental to the performance of the film in the final package.
For instance, too low free shrink values may result in a pack appearance unacceptable for the customer due to the looseness of the film and to the presence of wrinkles.
This is particularly true for the vacuum packaging of meat products, particularly fresh meat products. Upon evacuating the atmosphere from the package followed by heat-sealing of the film, the resulting closed package should tightly shrink around the meat product. A film endowed with a sufficiently high free shrink retracts against the product, reducing the excess of film protruding away from the packaged product and improving the appearance and the function of the package.
At this shrinking step, it is essential that the films develop proper free shrink values in both the directions together with an appropriate shrink force. This force must be high enough to tightly enclose the product within the film but without crashing or excessively distorting the final package. The free shrink and the maximum shrink tension, i.e. the maximum value of tension developed by the films during the heating/shrinking process, are thus parameters very important for achieving an optimal package appearance.
Another important requirement is that the packages should remain tight overtime during handling and storage.
In fact, one common inconvenient that occurs during the storage into the refrigerator is the so called “package relaxation”, namely the loss of pack tightness and the appearance of anti-aesthetical wrinkles and pleats in the packaging film. Package relaxation is not only undesirable for purely aesthetical reasons—the presence of wrinkles in the film of the package is not attractive per se—but also because it may impair the visual inspection of the packaged product and thus instill doubts concerning the freshness and the proper storage of the food. The measurement of the residual shrink tension at typical fridge temperatures can help to foresee any package relaxation and accordingly improve film properties.
Other important requirements of the package, for the consumer perception, are the optical properties, namely its transparency and its gloss. The transparency allows the consumer to “see through” the package and inspect the product and additionally, a glossy package is undoubtedly more attractive. Particularly in the case of barrier shrink films, where the barrier layer is for example EVOH or PVDC, the wrinkling of the barrier layer due to the high shrink of the film causes a significant worsening of the optics, especially in terms of the haze of the film.
For these reasons, it is crucial to preserve as much as possible excellent optical properties after the shrink, especially in the case of highly shrinking barrier films.
An improved stiffness of the film generally results in packages with less leakers leakers which are due to accidental openings or ruptures during the packaging process or handling of the packages. More rigid films also provide for an improved machinability, which allows decreasing the number of rejects and increasing the speed of packaging cycles. In fact, a film having good machinability is less subject to creasing, folding, seal pleats, edge curls, or jamming and can be more easily used with any packaging machine. Additionally, more stiff films provide for flexible containers which are easier to be loaded with the product and they generally show improved printability and converting.
Two main different strategies to impart stiffness have been applied in the art: film thickness was increased and abuse resistant resins, in particular polyesters and polyamides, were added. Both the strategies had some negative consequences in terms of process efficiency and costs.
Moreover, in case of shrinkable films, it has been observed that an increase in the stiffness often results in undesired shrinking properties and in worsened optical characteristics (e.g., gloss, haze before and after shrink).
The addition of abuse resistant resins, also herein named as “stiff resins”, in particular of high melting point polymers, such as polyamides or particularly aromatic polyesters, resulted in further issues.
First, when these films incorporate a barrier layer comprising PVDC, the different thermal behaviour and stability of the barrier polymers with respect to the abuse resistant resins, make the manufacturing process of the film very difficult.
In particular, films including high melting polyesters, such as aromatic (co)polyesters, together with a thermolabile PVDC barrier layer, would be hardly obtainable with conventional extrusion dies and/or processes, namely by co-extrusion of all layers—as taught in WO2005011978 or EP2147783—or by extrusion coating of a substrate with a coating, in which the coating comprises both PVDC and PET—as suggested in EP2030784.
The Applicant found out that by applying said conventional processes with traditional extrusion dies to the manufacture of the present films, there may occur so much damage of the PVDC layer that the final film would not be acceptable in terms of colour, oxygen transmission and/or optics. In fact, the Applicant observed that the temperatures applied to extrude polyamides and especially aromatic polyesters are generally so high—e.g. up to 270-280° C.—to induce partial degradation of the PVDC barrier layer, with appearance of undesired yellow brown colors and possible deterioration of gas barrier performance. In this regard, the state of the art provides little or no teaching on how to solve the problem of thermal incompatibility and how successfully manufacturing this kind of films.
Additionally, if the structure of the films is not symmetrical, incorporation of high amounts of abuse resistant resins may cause or worsen the so called “curling effect”, i.e. the tendency of the edges to roll up when the film or tubing are cut.
Of course, both at manufacturing and customer level it is most desirable that the tubing or the film stay flat when cut. When the tubing or web curls, it becomes really difficult (or even impossible) to run the standard converting operations like bag-making, slitting, printing, unfolding.
Curling is also a serious issue at customer level because it makes difficult running the bags on the automatic machines (bags loader, FIFFS machine, thermoform-shrink machine) and dramatically increases the rejects due to wrong bags opening and/or web positioning.
In addition, for complex film formulations comprising barrier layers (for example EVOH or PVDC) and some layers of stiff resins, such as polyesters and polyamides, the set-up of the extrusion process is more critical and requires several line adjustments before finding a good compromise of process yield vs. film properties.
Such formulations also generate other problems in terms of sealability and bond strength between the various layers of the structures.
There is still the need for new barrier heat shrinkable films endowed with good optical properties after shrink, very good sealability and appropriate bond strength between the various layers of the films, good processability and rigidity but reduced curling to ensure robustness/easy handling at converting and printing.